The present invention relates to thermoelectric heat exchangers; particularly, to an improved thermoelectric heat exchanger for transferring heat between two fluids.
Many types of industrial equipment require liquid cooling or heating during their operation. Typical examples include semiconductor process equipment, pharmaceutical and biotechnology fermentation/separation vats, machine tools, air conditioners, plastic molding/extrusion equipment, analytical equipment, welding equipment, and lasers. One common way to provide the required cooling or heating is with a recirculating coolant temperature control unit, or chiller. A typical chiller consists of a freon-based refrigeration loop connected to a recirculating coolant loop via a heat exchanger. However, as the world community becomes increasingly concerned about ozone depletion and global warming, a replacement for the standard freon-based refrigeration technology is urgently needed. Thermoelectric technology offers a clean, environmentally-friendly, solid state alternative.
Thermoelectric cooling was first discovered by Jean-Charles-Athanase Peltier in 1834, when he observed that a current flowing through a junction between two dissimilar conductors induced heating or cooling at the junction, depending on the direction of current flow. Practical use of thermoelectrics did not occur until the early 1960s with the development of semiconductor thermocouple materials which were found to produce the strongest thermoelectric effect. Most thermoelectric materials today comprise a crystalline quaternary alloy of bismuth, tellurium, selenium, and antimony.
Thermoelectric modules are manufactured by electrically connecting in series a large number of bismuth-telluride P-type (25% Bi.sub.2 Te.sub.3 +72% Sb.sub.2 Te.sub.3 +3% Sb.sub.2 Se.sub.3) and N-type (90% Bi.sub.2 Te.sub.3 +5% Sb.sub.2 Te.sub.3 +5% Sb.sub.2 Se.sub.3) crystal pairs. When direct current is passed through the module, heating or cooling at the junction of the dissimilar materials occurs depending on the direction of current flow. The junctions are physically arranged so that each cold junction in the series is in thermal contact with the body to be cooled and each hot junction is in contact with the body to be heated. Such an arrangement allows for the transfer of heat between the two bodies.
Although thermoelectric technology has been available since the early 1960s, most applications have employed the technology for cooling electronic devices or air. For liquids, the few current applications employ relatively inefficient means to transfer heat to, or from, the liquid coolant to another fluid. For example, current thermoelectric devices which inject their heat into ambient temperature (21.degree. C.) fluids can only cool liquids to near the freezing point of water.
A more effective thermoelectric heat exchanger is therefore desirable to transfer heat between two fluids. The improved heat exchanger should cool a liquid 40.degree. C. below the temperature of the second fluid without requiring the stacking of multiple thermoelectric modules within the heat exchanger, often referred to as cascading. The present invention uses compact heat transfer surfaces to minimize the resistance to heat transfer and thus allows for more efficient cooling to levels not before attained.